Bone marrow in boneless fish: lessons of evolution.

The existence of marrow tissue in cartilaginous ganoid fish, before the evolutionary development of bone, implies that the marrow and bone are two independent organs, developed separately and merged together by evolutionary pressure. Their relation is, therefore, matrimonial and not patrimonial. This should form a frame of reference for studying the interrelationship of bone and marrow. Some of the possible physiologic consequences of this merger are discussed. Evolutionary aspects of hemopoiesis have a potential for yielding information that may not otherwise be possible.